1. Field of the Invention
The present invention relates to a synchronizer sleeve adapted for use in a gear synchronizer mechanism of a power transmission for an automotive vehicle and a press-forming apparatus for the synchronizer sleeve, and more particularly to a press-forming apparatus for forming a chamfer on each distal end of inner spline teeth of the synchronizer sleeve by plastic deformation.
2. Discussion of the Prior Art
Disclosed in Japanese Patent Laid-open Publication No. 48-67649 is a synchronizer sleeve S adapted for use in a gear synchronizer mechanism of a power transmission as shown in FIGS. 1(A) and 1(B). The synchronizer sleeve S is formed at its inner periphery with circumferentially spaced inner spline teeth 1 to be meshed with a clutch gear of a change-speed gear train and inner spline teeth 2 for synchronization shorter in axial direction than the inner spline teeth 1 for meshing engagement and positioned among them in a circumferential direction to be meshed with a synchronizer ring.
In a conventional manufacturing process of the synchronizer sleeve, the inner spline teeth 1 for meshing engagement and the inner spline teeth 2 for synchronous engagement are formed by machining with a broach-cutter as shown in FIG. 2(A), and subsequently a chamfer 1a or 1b for meshing engagement is formed by an end-mill cutter on each distal end surface 3 of the inner spline teeth 1 indexed one by one tooth as shown in FIG. 2(B). Thereafter, the machining condition is changed as shown in FIG. 2(C), wherein a chamfer surface 2a or 2b for synchronous engagement is formed by a cutting tool such as an end-mill cutter on each distal end surface of the inner spline teeth 2 for synchronous engagement indexed as shown in the figure. After finish of the chamfer machining at one surface, the work piece of the synchronizer sleeve is reversed in position, and the chamfer machining is carried out by the cutting tool 4 such as the end-mill cutter in the same manner as described above as shown in FIGS. 2(D) and 2(E).
In such a machining process as described above, there will occur the following problems.    1) In the cutting process, the cutting time of the chamfers for meshing engagement and synchronous engagement inevitably becomes long, resulting in an increase of the manufacturing cost of the synchronizer sleeve S.    2) In operation of the power transmission, the synchronizer sleeve is moved by shifting operation of a driver toward a synchronizer ring N and a clutch gear P (a piece gear) from a neutral position as shown in FIG. 3(A). In such operation, an intersection 2d of the chamfer surface 2b and a spline tooth flank of the synchronizer sleeve S is moved in slide engagement with the chamfer surface 6 of synchronizer ring N so that the rotation speed is synchronized by frictional engagement of the synchronizer ring N with the clutch gear P. To avoid seizure of the synchronizer mechanism during the synchronous operation, a copper material lower in hardness than the material of the synchronizer sleeve S is used as the raw material of the synchronizer ring N. For this reason, if the intersection 2D of the chamfer surface 2b of synchronization and the spline tooth flank 2f of synchronizer sleeve S was not rounded off, the chamfer surface 6 of synchronizer ring N would be scraped at each shifting operation. This shorten the life span of synchronizer ring N and deteriorates the shift feeling in a short time.
In addition, when the synchronizer sleeve S is brought into meshing engagement with the clutch gear P as shown in FIG. 2(D) after passed through the synchronizer ring N as shown in FIG. 3(C) in the shifting operation, the chamfer ridge 1c of synchronizer sleeve S tends to abut against the chamfer ridge 7 of clutch gear P. In such an instance, if the chamfer ridge 1c was not rounded off, the chamfer ridge 1c would be damaged since the harness of the raw material of the synchronizer sleeve is the same as that of the clutch gear P, resulting in deterioration of the shift feeling. Accordingly, it required in the synchronizer sleeve S to round off the intersection 2e of the chamfer surface 2a for synchronization and the spline tooth flank 2f, the intersection 2d of the chamfer surface 2b for synchronization and the spline tooth flank 2f and the ridge 1c of the chamfer surfaces 1 for meshing engagement. It is, however, impossible to round off the intersections 2e, 2d and the chamfer ridge 1c by cutting as shown in FIGS. 2(A)–2IE). For this reason, the intersections 2e, 2d and the chamfer ridge 1c must be rounded off respectively at different steps, resulting in an increase of the manufacturing cost of the synchronizer sleeve.    3) In the synchronizer sleeve S whose inner spline teeth 2 for synchronization to be meshed with the synchronizer ring are formed shorter in axial length than the inner spline teeth 1 to be meshed with the change-speed gear and positioned among the inner spline teeth 1 as shown in FIGS. 1(A) and 1(B), each chamfer of the inner spline teeth 2 for synchronization is formed in a position displaced axially inwardly from the chamfers of inner spline teeth 1 for meshing engagement. In the case that the chamfers of inner spline teeth 2 are formed by cut-machining described above, the cutting work is restricted due to relative position of the chamfers of inner spline teeth 2 for synchronization to the chamfers of inner spline teeth 1 for meshing engagement. Accordingly, in a condition where the chamfer portion of inner spline teeth 2 for synchronization is spaced in a large distance from the chamfer portion of inner spline teeth 1 for meshing engagement, as shown in FIG. 4(A), the chamfers of inner spline teeth 2 for synchronization may not be formed by the cutting tool 4 such as the end-mill cutter since the cutting tool is interfered with the chamfers of inner spline teeth 1 for meshing engagement at a portion 8 in contact therewith. In such a case, it is obliged to reduce the space L between the chamfer portion of inner spline teeth 1 and the chamfer portion of inner spline teeth 2. This means that the synchronizer sleeve has to be designed at the sacrifice of shift feeling. To avoid such problems, proposed in Japanese Utility Model Laid-open Publication Nos. 49-83448 and 50-122732 is a synchronizer sleeve whose chamfers are formed asymmetrical in a circumferential direction. In the cutting process of the synchronizer sleeve, however, the end-mill cutter 4 is interfered with a chamfer 9 of an adjacent spline tooth, resulting in restriction of the offset amount of the chamfer ridge and the shape of the chamfer surface. This means also that the synchronizer sleeve has to be designed at the sacrifice of shift feeling.